Sorry no hard facts for you smalltown, but any 1688 I have ever been around was pretty strong, wouldn't be surprised if it wouldn't out run the 21. Was the max engine speed different between these two (higher on the 16)?
I have a buddy who just bought a 1996 2188, and he is dead set that it will run away from our 1994 1688... Our family has run CIH forever, 2388,1688,1680,1482 along with some older pull types. But i can't find the differences between 2188 and 1688... I thought maybe afew more horses in the 2188 but the cleaning surfaces are the same, and the motors were the same but with different injectors? I need hard facts here... I know a 2188 won't do anymore than a 1688 but i need hard facts.